Sheet processing machines, such as printing presses, require a continuous input of sheets at a constant rate. Typically, the sheets are provided to such sheet processing machines in the form of a sheet stack, whereby sheets are fed into the sheet processing machine from the top of the sheet stack via a feeding mechanism. In order for these feeding mechanisms to reliably engage the sheets, it is a necessary requirement that the uppermost sheet in the stack be maintained within a relatively narrow height range.
To keep the uppermost sheet within the required range, the sheets are typically stacked upon a stack lifting table which can be raised as needed to compensate for the reduction in the stack height as sheets are removed. Ordinarily, a motorized stack lift drive is provided for raising the stack lifting table and the stack of sheets thereon so that the top surface of the uppermost sheet is within the proper height range. Since the speed that sheets are fed to the sheet processing machine depends on the operating speed of the printing press, and since the height of the top of the stack varies in dependence on the sheet thickness as sheets are removed, the drive must raise the stack lifting table in dependence on these parameters.
One method of keeping the top of the stack within the appropriate range of heights utilizes an operator to manually adjust the rate at which the stack lifting table rises. With this method, an initial lifting speed is estimated from the sheet thickness and the operating speed of the sheet processing machine. The estimated lifting speed is then typically corrected by the operator based on observation and experience. This method is not very desirable, inasmuch as it requires operator attention to the sheet processing machine. The problem is particularly troublesome when sheet stacks having unknown sheet thicknesses are to be fed to the sheet processing machine, in which case the sheet thickness must be recalculated.
Other methods of operating lift motors to maintain the required stack height are described in U.S. Pat. No. 5,295,678, assigned to the assignee of the present invention and hereby incorporated by reference in its entirety. In the method and system of U.S. Pat. No. 5,295,678, the lift motor operates in either a continuous or discontinuous manner depending on the sheet thickness input into the system. While this system is generally reliable, it selects the operating mode of the motor according to the sheet thickness and processing speed. Proper motor operation thus depends on the proper sheet thickness being input into the system by an operator, a task which must be carefully repeated each time the type of sheet is varied. Moreover, if a sheet selected for measurement is particularly thick or thin with respect to average sheet thicknesses, the motor operating speed will be incorrect.